The human heart is a pump—a complex and critical pump. The heart can become damaged or dysfunctional over time. When wear and damage to the heart become sufficiently serious, the heart fails to pump and circulate blood normally, resulting in a condition known as heart failure. Around the world millions of people suffer from heart failure. Many people are unresponsive to pharmacological intervention and could benefit from a heart transplant. Since there is a shortage of donor hearts, blood pumps of different forms, including implantable ones, have gradually evolved into a viable treatment option.
In a diseased state, one or both ventricles of a patient's heart can become weakened to an extent that mechanical intervention to supplement circulation is needed to keep the patient alive. In extreme circumstances, the entire heart is removed and replaced with an artificial heart while in other cases a device that assists the heart is used. A blood pump system used to assist the heart without removing the natural heart is commonly referred to as a ventricular assist device.
Although either of the ventricles of the heart may function in a weakened state, failure of the left ventricle is more common. Normally, blood enters the left ventricle through the mitral valve and, during heart systole, the blood is ejected through the aortic valve and into the aorta by the squeezing action of the left ventricle. To assist a failing left ventricle, a ventricular assist device can be attached to the apex of the left ventricle supplementing blood flow between the left ventricle and the aorta. As a result, blood entering the left ventricle may either be ejected through the aortic valve by the ventricle or pass through the ventricular assist device into the aorta.
Ventricular assistance has been performed by a variety of blood pump designs. The majority of the early ventricular assist devices, such as positive displacement pumps, pumped blood in a pulsatile manner. In this case, the ventricular assist device allows an internal sac to passively fill with blood, and then utilizes pneumatic action to compress the internal sac, ejecting the blood into the patient's aorta to supplement circulation.
These pulsatile ventricular assist devices are large and can only be used as an implantable treatment option for patients with a large body surface area.
To overcome the size and complexity problems associated with the pulsatile ventricular assist devices, designers have begun to use continuous flow pumps. These pumps are smaller than their pulsatile counterparts and are more reliable. Continuous flow or rotary pumps are normally either centrifugal flow pumps or axial flow pumps. In the centrifugal flow pumps, the rotors are shaped to accelerate the blood circumferentially and thereby cause it to move toward the outer rim of the pump, whereas in the axial flow pumps the rotors are cylindrical with helical blades, causing the blood to be transported in the direction of the rotor's rotational axis. Alternative designs exist spanning these two general types and are termed diagonal or mixed flow pumps. A main advantage of axial flow pumps over centrifugal flow pumps is that they are smaller for a given fluid displacement capacity.